Semiconductor nanoparticles whose particle sizes are 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules. Their physicochemical properties are therefore different from both bulk semiconductor crystals and molecules. In this region, due to the quantum-size effect, the energy gap of semiconductor nanoparticles increases as their particle sizes decrease. In addition, the degeneration of the energy band that is observed in bulk semiconductors is removed and the orbits are dispersed. As a result, the lower-end of the conduction band is shifted to the negative side and the upper-end of the valence band is shifted to the positive side.
Semiconductor nanoparticles of CdS can be easily prepared by dissolving equimolar amounts of precursors of Cd and S. This is also true for the manufacturing of, for example, CdSe, ZnS, ZnSe, HgS, HgSe, PbS, or PbSe.
Semiconductor nanoparticles have drawn attention since they emit strong fluorescences whose full widths at half maximum are narrow. Thus, various fluorescent colors can be created, and their future applications can be nearly infinite. However, the semiconductor nanoparticles obtained only by mixing the precursors with each other as described above have a wide distribution of particle sizes and therefore cannot provide the full advantage of the properties of semiconductor nanoparticles. Attempts have been made to attain a monodisperse distribution by using chemical techniques to precisely separate and extract only the semiconductor nanoparticles of a specific particle size from semiconductor nanoparticles having a wide distribution of particle sizes immediately after preparation. The attempts to attain a monodispersed distribution of particle sizes that have been reported so far include: separation by electrophoresis that utilizes variation in the surface charge of nanoparticles depending on their particle sizes; exclusion chromatography that utilizes differences in retention time due to different particle sizes; and size-selective precipitation that utilizes differences in dispersibility in an organic solvent due to differences in particle sizes.
A method was described above wherein the nanoparticles, which were prepared by mixing the precursors with each other, were separated depending on their particle sizes. Also reported is size-selective photoetching that attains a monodispersed distribution of particle sizes by utilizing the oxidative dissolution of a metal chalcogenide semiconductor in the presence of dissolved oxygen when irradiated with light.
There is also a method wherein a monodispersed distribution of particle sizes is attained through regulation at the phase of mixing the precursors with each other. A representative example thereof is the reversed micelle method. In the reversed micelle method, amphiphilic molecules such as diisooctyl sodium sulfosuccinate are mixed with water in an organic solvent such as heptane to form a reversed micelle therein, and precursors are allowed to react with each other only in an aqueous phase in the reversed micelle. The size of the reversed micelle is determined according to the quantitative ratio of the amphiphilic molecules to water, and its size can be relatively homogenously regulated. The sizes of prepared semiconductor nanoparticles depend on the size of the reversed micelle. Thus, semiconductor nanoparticles with relatively homogenous particle sizes can be prepared.
The thus prepared semiconductor nanoparticles exhibit a relatively narrow distribution of particle sizes. However, the fluorescence properties of the thus prepared semiconductor nanoparticles show smooth fluorescence emission spectra without any significant peaks. Further, the fluorescence emission spectra exhibit a peak at a wavelength that differs from the theoretical value of the fluorescence, which should be emitted by the semiconductor nanoparticles. Specifically, in addition to the band gap fluorescence emitted inside the semiconductor nanoparticles, the semiconductor nanoparticles emit completely different fluorescences, which are supposed to be emitted at the energy level in the forbidden band in the semiconductor nanoparticles. The energy level that emits this fluorescence is considered to exist mainly at the surface site of the semiconductor nanoparticles. Under normal circumstances, changes in fluorescence properties due to the particle size regulation of the semiconductor nanoparticles appear in the band gap fluorescence. This inhibits the properties of semiconductor nanoparticles having a narrow distribution of particle sizes, and thus, it has been a problem that should be solved. As a representative solution for this problem, a method has been attempted wherein a semiconductor material, which is a nucleus, is coated with a semiconductor material, inorganic material, or organic material having a band gap wider than that of the aforementioned semiconductor material to attain a multi-layer structure, and fluorescences thereof are inhibited. Examples of particularly representative methods of coating the semiconductor nanoparticles with inorganic materials include: coating of CdSe nanoparticles with CdS (J. Phys. Chem. 100: 8927 (1996)); coating of CdS nanoparticles with ZnS (J. Phys. Chem. 92: 6320 (1988)); and coating of CdSe nanoparticles with ZnS (J. Am. Chem. Soc. 112: 1327 (1990)). Regarding the CdSe nanoparticles coated with ZnS (J. Am. Chem. Soc. 112: 1327 (1990)), with the utilization of the Ostwald ripening, a production method that is carried out in the coordinated solvent is adopted, thereby successfully obtaining semiconductor nanoparticles having sufficient fluorescence properties (J. Phys. Chem. B. 101: 9463 (1997)). The aforementioned multi-layered semiconductor nanoparticles inhibit the defective site on the surface of the semiconductor nanoparticles and attain the original fluorescence properties of the semiconductor nanoparticles by coating them with a material having a band gap larger than that of the semiconductor nanoparticles and having no band gap in the forbidden band of the semiconductor nanoparticles.
Meanwhile, improvement in fluorescence properties of semiconductor nanoparticles in an alkaline aqueous solution has been reported as a physical property of the semiconductor nanoparticles in an aqueous solution (J. Am. Chem. Soc. 109: 5655 (1987)). Various experiments and reports have been made based on this report. However, none of them has arrived at an elucidation of the mechanism involved (e.g., J. Phys. Chem. 100: 13226 (1996) and J. Am. Chem. Soc. 122: 12142 (2000)). All the semiconductor nanoparticles in the alkaline solution were poor in reproducibility, and conditions for reproduction have not yet been identified. Further, none of these experiments or reports has succeeded in isolating a final substance.
The thus obtained semiconductor nanoparticles can inhibit the defective site to some extent, and have the original properties of the semiconductor nanoparticles to a certain extent. However, an advanced technology is necessary in order to prepare such semiconductor nanoparticles. In order to prepare semiconductor nanoparticles of higher quality, various facilities are required. In addition, from the viewpoints of a cost of reagents, safety of reaction at high temperature, and other factors, there have been serious drawbacks in terms of industrial mass-production.
Compared to currently used reagents such as organic pigments, however, semiconductor nanoparticles are more durable and fade less readily. Further, variation in particle sizes enables the development of various reagents, which exhibit spectra whose full widths at half maximum are narrow, from the same ingredient. This enables the application thereof not only to the optical devices and the like but also to, for example, the detection of biopolymers and the bioimaging. This indicates that the applications thereof are nearly infinite. Accordingly, semiconductor nanoparticles have recently drawn attention, and finding solutions to these drawbacks has been a recent object of researchers.